Three endpoints in history_routes.py ordered by
DbChatMessage.created_at, but the ChatMessage model has no
created_at column — only timestamp. This caused AttributeError
(HTTP 500) on mark-stopped, update-last-meta, and
merge-last-assistant. Other queries in the same file already use
the correct column.
Fixes#1659
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>